History of Geophysical Research in The Netherlands ... - DWC - KNAW

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phere. On the other hand, properties of the ionosphere are determined in great part by the existence of the geomagnetic field in the ionospheric layers. About 1920 some Dutch scientists, among whom the astronomer A. Pannekoek (1926) and the physicist G. J. Elias (1923), made calculations on the formation of ionization in the up per atmosphere, caused by the ultraviolet light of the sun. Elias especiaUy studied the propagation of electromagnetic waves in a partly ionized layer. Interest in the ionosphere was stimulated by the desirability of having radio contact bet ween the Netherlands and its Colonies . Already in 1923 a telegraphic connection had been made possible between Kootwijk (Netherlands ) and Malabar (Java) using very long radio waves. Later on it was shown that radio waves of relatively short wavelength were more suitable for covering great distances, by means of reflections from the ionosphere. Two scientists played a very important role in this field, viz. Balth. van der Pol (University of Technology at Delft) and H. Bremmer (University of Technology at Eindhoven). Both also worked with the Philips Research Laboratories at Eindhoven. Van der Pol carried out measurements of the electrical conductivity and the dielectric constant of an ionized gas, as a model of the ionosphere. The influence of a plasma on the propagation of radio waves was discussed in his doctor's thesis (Van der Pol, 1920). His early work was devoted especially to the mathematical aspects of radio science. However, upon his request, the program of the Netherlands expedition at Angmagssalik (see: section IV.l. International Polar Year, 1932-1933) also included soundings of the ionosphere at regular times. Bremmer , who joined Van der Pol's group, wrote many papers on tropospheric and ionospheric propagation of radio waves. He also dealt with phenomena such Figure 11. Sounding the ionosphere by reflection of radio waves of varying freq uency. The time delay bet ween transmitted and reflected pulses determines the height of the reflecting layer. By day three layers are present: the E-Iayer or Heaviside-Iayer, and above it the Fl-1ayer and the F2-layer (or Appleton-Iayer). The frequency of the reflected radio signa Is is a measure of the ionization at the height of reflection. The magnetic double refraction of radio waves in the ionosphere is shown by repetition of the Fl- and F2-reflections at higher frequencies (ionogram of KNMI, af ter Veldkamp, 1974; see also: Veldkamp and Scholte, 1954a). 24
as whistlers, ionospheric winds, tidal movements in the ionosphere and double refraction of radio waves. Summaries of his work are given in his book "Terrestrial Radio Waves: Theory of propagation" (Bremmer, 1949), and in the Encyclopaedia of Physics: "Propagation of electromagnetic waves" (Bremmer, 1958). Veldkamp , who had become director of the geophysical department of the KNMI in 1945, believed that study of the highest atmospheric layers should be included in the task of this Meteorological Institute, as weIl as continuous monitoring of the troposphere and the stratosphere. Together with H.J. Groenewold he commenced a regular study of the ionosphere in 1949 by means of a transmitter and receiver made in the "Institut für Ionensphärenforschung" at Lindau (Germany). After some years a new apparatus (with less mechanically moving parts and therefore greater reliability) was constructed by H. J. A. Vesseur (1977). Day and night the structure and the ionization of the ionosphere was and still is recorded every half hour by means of this "ionosonde". The ionosp here behaves like a mirror reflecting radio waves. Generally three layers (E, Fl and F2) are present in the ionosphere above the Netherlands, at heights of 110 km, 200 km and 300 km respectively. These layers are subject to regular variations between day and night, and also to irregular variations due to the activity of the sun. The aim of the soundings is to monitor continuously the highest layers of the atmosphere . The data acquired from the ionosphere are partly of purely scientific and partly of practical importance. Groenewold (1948) wrote a paper JANUARY 1970 Figure 12. Vector diagram of the wind in the E-layer of the ionosphere above De Bilt (average diurnal variation in January 1970). A line from the origin to the point marking a certain hour gives the average wind vector for that hour. The diagram shows that the wind vector in the E-layer rotates twice per day. In addition the prevailing wind is southwesterly during the whole day (Vesseur, 1977). 25 N
Page 1: History of Geophysical Research in
Page 4 and 5: ISBN 0-4448-5615-3 Author's address
Page 6 and 7: Section 11.3.3.1 Southeast Asia and
Page 8 and 9: Preface During the Congress of the
Page 11 and 12: CHAPTER I Research in Geomagnetism
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Page 16 and 17: .... Ol Figure 4. The KNMI during t
Page 19 and 20: Geomagnetic Observatories and Surve
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Page 26 and 27: Ftgure 13. The changing ionosphere
Page 29: effect discovered by Clay was confi
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Page 47 and 48: As, J.A. (1973) - The compensation
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(1972) tried to find arelation bet
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Collette, B.J. and K. W. Rutten (19
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Ritsema, A.R. (1962) - Pand S ampli
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Visser, S.W. (1937) - Aardbevingen
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CHAPTER 111 Research in gravity lII
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Submarine KIl KXIII KXIII K XVIII V
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the oceans. This is a consequence o
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shaped when a southeasterly crustal
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- - o - .tJ_- 7
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Veldkamp, J. (ed.) (1969) - Gravity
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a year at Angmagssalik (East Greenl
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Outside the Netherlands geophysical
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Figure 47. Launching of a meteorolo
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means of data from both seismology
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IV.6. WIND MEASUREMENTS IN THE IONO
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y W. Bosrna, S. B. Kroonenberg, R.
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Holland in 1953 to 1958. Only after
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depth of 250 to 350 meters, under t
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A major improvement in the method w
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T I M E I N 5 E C o N D 5 l2t 381 2
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tally in a processing centre. The w
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Dijkstra, J. en A. Volker (1957) -
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Koefoed, O. (1968) - The applicatio
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Veldkamp , J. (1951) - Geomagnetic
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versity of Toronto. There he was su
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